Re: [PATCH V2 2/3] memcg: alter mem_cgroup_{update,inc,dec}_page_stat() args to memcg pointer

[Michal Hocko <mhocko@suse.cz>]

On Tue 14-05-13 17:00:08, Sha Zhengju wrote:
> On Mon, May 13, 2013 at 8:25 PM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Mon 13-05-13 13:05:24, Sha Zhengju wrote:
> >> From: Sha Zhengju <handai.szj@taobao.com>
> >>
> >> Change the first argument of mem_cgroup_{update,inc,dec}_page_stat() from
> >> 'struct page *' to 'struct mem_cgroup *', and so move PageCgroupUsed(pc)
> >> checking out of mem_cgroup_update_page_stat(). This is a prepare patch for
> >> the following memcg page stat lock simplifying.
> >
> > No, please do not do this because it just spreads memcg specific code
> > out of memcontrol.c. Besides that the patch is not correct.
> > [...]
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1109,12 +1109,24 @@ void page_add_file_rmap(struct page *page)
> >>  {
> >>       bool locked;
> >>       unsigned long flags;
> >> +     struct page_cgroup *pc;
> >> +     struct mem_cgroup *memcg = NULL;
> >>
> >>       mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> >> +     pc = lookup_page_cgroup(page);
> >> +
> >> +     rcu_read_lock();
> >> +     memcg = pc->mem_cgroup;
> >
> > a) unnecessary RCU take for memcg disabled and b) worse KABOOM in that case
> > as page_cgroup is NULL. We really do not want to put
> > mem_cgroup_disabled() tests all over the place. The idea behind
> > mem_cgroup_begin_update_page_stat was to be almost a noop for !memcg
> > (and the real noop for !CONFIG_MEMCG).
> 
> It's indeed an unwise behavior. How about also wrapping it in
> mm/memcontrol.c or memcontrol.h?

I just think it doesn't make much sense to overcomplicate this -
especially when this is not an issue anymore.
-- 
Michal Hocko
SUSE Labs

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